Field of the Invention
The present invention relates to an electrochromic element and a method of driving the same, and further, to an optical filter, a lens unit, an image pickup apparatus, and a window member using the electrochromic element.
Description of the Related Art
An electrochromic (hereinafter sometimes abbreviated as “EC”) element that uses an EC material as a substance that undergoes changes in optical absorption properties (absorption wavelength and absorbance) through an electrochemical redox reaction is applied to a display apparatus, a variable reflectance mirror, a variable transmission window, and the like. In such an EC element, suppression of change in optical characteristics as time progresses is one of the biggest challenges. In particular, in an EC element called a complementary EC element that uses an anodic EC material and a cathodic EC material, insufficient decoloring due to charge imbalance is a problem. Note that, the concept of charge balance/imbalance is described later.
In U.S. Pat. No. 6,188,505, there is disclosed use of a non-EC material that is more easily oxidized than an anodic EC material and a non-EC material that is more easily reduced than a cathodic EC material (those materials are hereinafter referred to as “redox buffers”) in a complementary EC element in which the EC materials are dissolved in an electrolyte. In the EC element disclosed in U.S. Pat. No. 6,188,505, an oxidized form and a reduced form of the redox buffers are more stable than an oxidized form as a colored form of the anodic EC material and a reduced form as a colored form of the cathodic EC material, respectively. Therefore, even when charge imbalance occurs in decoloring operation, insofar as the amounts of charge of the redox buffers can cover, generation of the oxidized form and the reduced form of the corresponding redox buffers are dominant over remaining of the colored forms of the EC materials. The redox buffers are non-EC materials, and thus, even when the oxidized form and the reduced form of the redox buffers are generated, the redox reactions thereof do not affect the transmittance of light. In other words, the redox buffers add a charge balance region in which the color does not fluctuate so that the charge imbalance of the EC element does not directly result in insufficient decoloring.
However, in U.S. Pat. No. 6,188,505, the redox buffers are more easily oxidized than the anodic EC material or more easily reduced than the cathodic EC material, and thus, are more liable to react in terms of the potential than the EC materials. Therefore, in normal coloring operation of the EC element, the redox buffers react ahead of (at least equivalent to) the EC materials. As a result, there are problems in that, compared to a case in which the redox buffers are not used, a current that does not contribute to the coloring unnecessarily flows to increase the power consumption and to reduce the response speed.
Further, even when the redox buffers are used as in U.S. Pat. No. 6,188,505, charge imbalance between display electrodes is not eliminated. Specifically, only the colored forms of the EC materials are reduced (instead, the oxidized/reduced forms of the redox buffers that are not colored or decolored are generated), and the charge balance between the display electrodes is not affected. When charge imbalance occurs in a complementary EC element, the colored form ratio of the anodic EC material/the cathodic EC material changes. Specifically, the ratio of coloring of a material having a polarity opposite to that of a material remaining due to charge imbalance becomes smaller than the ratio of coloring of the material remaining due to charge imbalance. For example, when the EC element is colored from a charge imbalance state in which a colored form of the cathodic EC material remains, compared to a case in which charge imbalance does not occur, the ratio of coloring that comes from the anodic material is smaller than the ratio of coloring that comes from the cathodic material. As a result, an actual absorption spectrum different from an absorption spectrum that is assumed at design time appears as discoloration of a color absorbed by the EC element, which is not preferred. In U.S. Pat. No. 6,188,505, in decoloring operation, charge of an oxidized form of the anodic material or a reduced form of the cathodic material that remains due to charge imbalance is undertaken by the redox buffers to suppress remaining of the color of one polarity of the anodic material or the cathodic material. However the charge imbalance between the display electrodes is not corrected, and thus, the colored form ratio of the anodic EC material/the cathodic EC material cannot be corrected. In other words, even if charge imbalance between the display electrodes occurs, the color is not seen in decoloring operation, but, when coloring is performed again, a state in which the ratio between the anodic material and the cathodic material changes from the initial one state due to the imbalance appears.